Why not really? Because all of those Merlin variants feature different fuselages, with differences such as more/different windows, different doors or downright differently shaped parts. You can, however, use specific parts and equipment of these as tuning.
Merlin MK.2: External Rearview Mirrors; ASW sensors; Torpedos
CH-149 Cormorant: Search and Rescue Equipment
FAP AW-101: SAR Equipment, FLIR on the front and some additional minor parts around the fuselage
Additionally you can equip an aerial refueling probe.
The rear interior can be completely walked in and you’re able to transport small vehicles. The sliding side doors and rear ramp are openable, by opening bomb bay doors you can extend the winch on the right side. Working weapons include Flares by default and Torpedos when equipped via tuning (note that these will still behave like missiles, but they can only lock-on to enemy occupied water vehicles)
The cockpit is detailed and you can seat 12 people inside. Please note that entering is done via warping, there is NO animation.
Credits:
SkylineGTRFreak